Man-made structures such as boat hulls, buoys, drilling platforms, oil production rigs, and pipes which are immersed in water are prone to fouling by aquatic organisms such as green and brown algae, barnacles, mussels, and the like.
Such structures are commonly of metal, but may also comprise other structural materials such as concrete or wood. This fouling is a nuisance on boat hulls, because it increases frictional resistance during movement through the water, the consequence being reduced speeds and increased fuel costs. It is a nuisance on static structures such as the legs of drilling platforms and oil production rigs, firstly because the resistance of thick layers of fouling to waves and currents can cause unpredictable and potentially dangerous stresses in the structure, and, secondly, because fouling makes it difficult to inspect the structure for defects such as stress cracking and corrosion. It is a nuisance in pipes such as cooling water intakes and outlets, because the effective cross-sectional area is reduced by fouling, with the consequence that flow rates are reduced.
It is known to use antifouling coatings, for instance as a top coat on ships' hulls, to inhibit the settlement and growth of marine organisms such as barnacles and algae, generally by release of a biocide for the marine organisms.
Many successful antifouling coatings in recent years have been “self-polishing copolymer” paints based on a polymeric binder to which leaving group moieties are chemically bound, and from which leaving group bonds to the polymer backbone are gradually hydrolysed by seawater. In such binder systems, the side groups of a linear polymer unit are split off in a first step by reaction with seawater, the polymer framework that remains becoming water-soluble or water-dispersible as a result. In a second step, the water-soluble or water-dispersible framework at the surface of the coating layer on the ship is washed out or eroded. Such paint systems are described for example in GB-A-1 457 590, EP779304, WO2005005516, WO200202698, WO2004018533 or WO201018144 and WO9937723.
Current marine solvent-borne antifouling coatings release significant volumes of organic solvent into the atmosphere upon coating application. This is harmful to the environment and will be legislated against increasingly in the coming years. The volatile organic content (VOC) of antifouling coatings is already restricted by legislation in many countries. For example, the European Solvents Emissions Directive limits the VOC to less than 270 g/kg—equivalent to about 450 g/l—for the overall emissions from shipyards which apply antifouling coatings compositions, whilst in California, USA an upper limit of 400 g/l for antifouling paints is in force.
Waterborne formulation offers the potential to significantly reduce the organic solvent content of antifouling coatings. For the purposes of the present invention, a waterborne antifouling coating composition means any such composition containing more than 10% of water by volume. Such coatings can also be referred to as water-based coatings. The waterborne compositions disclosed in this patent application typically have VOC<150 g/l.
Besides these VOC demands, antifouling coatings on ships should provide adequate antifouling protection and have a good film integrity, i.e. show almost no cracking and a good adhesion, particularly when applied to those parts of a vessel where the coating is alternately wet and dry, for instance at the waterline. The coating should be sufficiently hard, i.e. not soft or sticky, but it should not be brittle. Further, the coating should show hardly any so-called cold flow or plastic deformation, in other words, the film should not ripple when the ship moves in the water. Additionally, the coating composition needs to show a sufficiently short drying time. These characteristics are achieved by the compositions of the present invention.
JP2009173914 and WO2006/077738 and EP1958991 disclose waterborne antifouling systems prepared by latex polymerisation.
JP2007023243 discloses an aqueous antifouling coating composition which comprises an emulsion prepared by (i) preparing a polymer in an organic solvent, (ii) partially removing the solvent by distilling at atmospheric pressure and then (iii) mixing the polymer/solvent mixture with water. The emulsions that are described in JP2007023243 are water-in-oil emulsions. The reported viscosities of the emulsions in JP2007023243 vary between 1-6 poise, which is characteristic of a water-in-oil emulsion where the overall viscosity is controlled by the viscosity of the continuous phase (i.e. the polymer dissolved in the organic solvent). In contrast, water-in-oil emulsions (which have not been modified by addition of thickening agents) have a viscosity of around 0.1 poise (since water is the continuous phase). Further, it has been found by the inventors that the emulsions prepared as described in Examples P1 and P3 are unstable at ambient temperature when stored overnight.
JP2003277680 discloses a silyl polymer based system made via the synthesis of the silyl polymer in an organic solvent. This solvent is partially removed by distillation before emulsification of the silyl polymer into water.
A problem with current waterborne antifouling systems is that there is a reduction in mechanical strength and film properties when compared to solvent-borne systems. The particulate nature of these systems is not favourable to film coalescence resulting in voids in the dried paint film.
The problems noted above have been addressed by the waterborne coating of the current invention, which releases significantly less organic solvent into the atmosphere whilst maintaining good mechanical properties comparable to solvent-borne systems. Not only is this better for the environment but is beneficial to users of such paint, who will see the positive effects of reduced organic solvent emissions, both with respect to their successful compliance with regulations (avoidance of punitive fines), and with respect to the environmental impact and profile of their organisation.